Friday, February 11, 2011

Spinning up to speed

kw: weather phenomena, tornadoes

I am reading a book about storm chasing. I expect to review it tomorrow or the next day. As a preliminary, I'll review some technical points about tornadoes here, based on the tornado intensity scale developed by Ted Fujita, from F0 to F5. The wind speed cutoffs for each increase in intensity are
  • F0-F1: 73 mph = 117 kph = 33 m/s
  • F1-F2: 112 mph = 180 kph = 50 m/s
  • F2-F3: 157 mph = 252 kph = 70 m/s
  • F3-F4: 207 mph = 333 kph = 93 m/s
  • F4-F5: 261 mph = 420 kph = 117 m/s
  • Max measured: 318 mph = 512 kph = 117 m/s
Three of these are illustrated below. All the images are shown rather small; clicking on any of them will bring up a larger version.

An F0 tornado from Scenic Reflections. Most tornadoes are in this category. This kind of storm most "looks like a tornado", the tapering rope shape with a dust cloud at the bottom. They are seldom strong enough for the entire rotating wind funnel to fill with dust, though you can see dust wrapping around the funnel cloud for about half its height.

Stronger tornadoes look more menacing because they widen out with enwrapped dust. The funnel cloud is just the low-pressure center, where the cooling caused by the low pressure causes moisture to condense. Tornadoes only form in moist air, so the funnel cloud is a universal feature, whether visible or hidden by dust or dirt and debris.

An F2 tornado from TopCities. At ground level, the dust cloud is probably a quarter mile (0.4 km) across. With winds that exceed 100 mph by quite a bit, such a tornado will at least remove the roof of any house it happens to hit, and usually take down the walls also. While an F0 tornado is unlikely to flip a car over, an F2 will certainly flip it over, and may roll it quite a distance.

From here up the scale, tornadoes get more wedge-like. It takes a lot of suction to hold the funnel together as the wind speed rises, and the diameter grows proportionally. Tornadoes F2 and larger often have multiple vortexes, which will be discussed below.

An F5 tornado from Weatherzone. These are the insane kings of weather trouble. Wider at the base than they are tall, they are wedge-shaped and the debris funnel hides multiple "suction vortexes", as Dr. Fujita called them. The largest F5 track known was more than two miles (3+ km) wide.

These don't just roll cars around, they fling them like paper balls. I have seen an auto that spent a little time in an F4 or F5 tornado. It exemplified one weatherman's advice, "Get out of your car. When the tornado is finished with it, there won't be room inside it for you." The Chrysler was crumpled to the size of a Smart Car.

Hiding in a basement is not a certain path to surviving an F5, though it is your best shot. They have been known to clean out a basement, even ripping out part of the concrete walls.

A multiple-vortex tornado, probably of intensity F4, from The complex nature of the storm helps us understand how they can generate such high wind speeds, such as the 318 mph noted earlier. The major rotation of the storm is about 100-150 mph. This is the ground speed of the suction vortexes when the storm itself is stationary. The whirling of the suction vortexes about their own axes is another 75-150 mph. The outside edge of a suction vortex thus reaches 175-300 mph relative to the ground. Then when the entire storm picks up horizontal velocity, it can zip along at 25-50 mph, so total wind speeds of up to 350, and perhaps greater, are possible.

If you must try to survive a major tornado, the safest shelter is one of the totally buried steel shelters. I've never heard of a storm uprooting one of these. They are expensive, however, and when I lived in Oklahoma, we knew of only two people who had one. The next best is a specially built storm shelter in a basement room or near the center of a house. An above-ground storm shelter, however, is unlikely to ride out an F4 or F5 storm. Some things, you simply can't afford to prepare for. Then, your best defense is to be elsewhere. Anyone living in Tornado Alley who doesn't have a weather radio is a statistic waiting to be recorded.

1 comment:

Anonymous said...

Just so you know, the picture you use as an example of an F5 is not an F5 tornado. It was taken in 2010, as can be seen on the photo, and there were no tornadoes rated EF5 (F5 is no longer in use and has been replaced with EF5) in 2010.

Second, while F5 tornadoes are often wedge shaped, they can be any size and shape, and some can be very narrow and skinny (eg Elie, Manitoba or Sherman, TX).

Third, the picture of the multiple vortex tornado which you describe as "probably of intensity F4" is the Jarrell, Tx tornado of 1997. It was an F5 tornado, and left some of the most severe tornado damage ever recorded.